Modern investment casting procedures are frequently used to produce castings which have complex hollow interiors. Illustrative examples of such cast articles are cast turbine blades and vanes of a gas turbine engine wherein the blades/vanes include a complex hollow interior for conducting cooling air through the blade/vane interior during use in the hot turbine environment. The hollow interior of the blade/vane may comprise one or more passageways that are formed in the airfoil and root to conduct air through for cooling purposes during use in the turbine.
Such complex interiors are formed in the blade/vane by positioning a suitably configured ceramic core in the investment casting mold and solidifying the molten metal in the mold about the core. The core is removed from the solidified casting by leaching or other means, leaving a casting having a hollow interior corresponding to the configuration of the core.
Typically, the core is provided with "prints" at one or both ends located beyond the pattern portion defining the internal wall of the part or article to be cast so that these prints will be embedded in the ceramic material invested about the pattern/core during the mold formation operation. The core "prints" are not disposed in the mold cavity where the casting is solidified.
As the performance requirements for turbine blades/vanes have increased, the cooling requirements and thus the complexity of the internal passageways formed in the part have become more complex. This has necessitated use of even more complex cores.
A problem has been experienced in casting some turbine blades/vanes when there is movement or shift of the core in the mold during removal of the pattern material, during preheating of the mold prior to pouring the molten metal therein, and during casting of the molten metal into the mold. For example, during casting, the core can exhibit a temperature profile along its length that causes unwanted core movement. In particular, even a slight core displacement during pattern removal, during mold preheating, and/or during metal pouring has been found to result in unacceptable variations in the wall thickness of the hollow cast blade/vane, especially when relatively thin ceramic cores are used and especially when single crystal or directionally solidified castings are formed in a mold heated to an elevated temperature prior to metal pouring and kept in this condition for a long period of time during solidification of the molten metal.
The use of relatively thin, complex cores presents additional problems as a result of warpage oftentimes associated with such thin cores. In particular, certain regions of these cores become warped during a firing operation employed in their manufacture and during the subsequent processing as described above (e.g., during pattern removal, mold preheating and metal pouring). Such warpage can ultimately produce unacceptable wall thickness variations in the hollow casting made therewith.
Various attempts have been made to provide means for accurately supporting cores in an investment casting mold. For example, chaplets such as described in U.S. Pat. No. 2,084,247 represent well-known prior core supporting techniques. Other techniques specifically developed for use in connection with ceramic molds/cores are set forth in U.S. Pat. Nos. 3,596,703; 3,659,645; 4,487,246; and 4,811,778. Some of these techniques use platinum chaplets, pins and similar devices extending through the wax pattern into contact with the core at one end and into the mold wall at the other end to position the core in the mold. However, these techniques create a problem of unwanted metal on the casting surfaces where the chaplets/pins extend into the mold wall. In effect, the molten metal cast into the mold eventually fills the space occupied by the chaplet/pin in the mold wall. This problem leads to the requirement of additional mechanical finishing operations to remove the unwanted metal, dimensional control variations, and possible unwanted nucleation/ recrystallization.
The aforementioned U.S. Pat. No. 3,596,703 describes a prior core positioning technique wherein holes are drilled in the wax pattern formed about the core until the holes reach the core. The ceramic mold is then invested about the wax pattern/core assembly so that ceramic material fills the drilled holes to provide ceramic plugs for supporting the core in the mold when the wax pattern is subsequently removed. When molten metal is cast and solidified in the mold, holes are left on the casting where the ceramic support plugs existed and are then plugged or removed. This technique involves laborious and costly hole drilling operations in the wax pattern and hole filling/removal operations on the casting.